An electrode mixture is used for electrodes in a nonaqueous electrolyte secondary battery and the like. A typical example of the nonaqueous electrolyte secondary battery is a lithium secondary battery. The lithium secondary battery has already been put into practical use as small-size power sources for portable telephones, notebook-sized personal computers, and the like, and further is being attempted to be applied in large-size power sources of automobiles, electric power storages, and the like.
The lithium secondary battery generally includes a positive electrode containing a positive electrode active material capable of being doped and dedoped with lithium ions, a negative electrode containing a negative electrode active material capable of being doped and dedoped with lithium ions, and an electrolyte.
The electrode includes an electrode collector and an electrode mixture mounted on the electrode collector. In order to mount the electrode mixture on the electrode collector, an electrode mixture paste containing the electrode mixture and a dispersion medium is used. Examples of the electrode mixture paste include pastes obtained by mixing and kneading electrode active materials such as a positive electrode active material and a negative electrode active material, a binder and a dispersion medium. Examples of the mixture of the binder and the dispersion medium include organic solvent-based binders that are solutions and the like obtained by dissolving polyvinylidene fluoride (binder) in N-methyl-2-pyrrolidone (dispersion medium). Meanwhile, in order to reduce the production cost up of electrodes, which is caused by the use of an organic solvent, it is known that a mixture of a binder and water (hereinafter, which is also referred to as a water-based binder) is used as the mixture of the binder and the dispersion medium. Specific examples of the water-based binder, which are known, include an aqueous dispersion of polytetrafluoroethylene (JP2-158055A), and an aqueous solution obtained by dissolving a water-soluble macromolecule such as carboxymethyl cellulose or hydroxyethyl cellulose in water (JP2002-42817A and JP2004-342517A).
Incidentally, when the positive electrode active material is a lithium-containing metal composite oxide mainly including lithium nickelate or nickel, the battery capacity of the lithium secondary battery can be increased. However, when the water-based binder such as the aqueous dispersion is used instead of the organic solvent-based binder, water and the positive electrode active material are reacted with each other, causing problems such as the reduction of the discharge capacity of the resultant lithium secondary battery, and the reduction of the charge and discharge cycle characteristics. The problems become remarkable when the lithium secondary battery is discharged with a large amount of electric current.